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Thread: Multotec Wear Linings Controls Wear with Ceramics

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    Multotec Wear Linings Controls Wear with Ceramics

    Wear caused by aggressive operating conditions in heavy industrial applications can be effectively countered using engineered ceramics, which in certain cases have shown vastly improved results over metals or polymers. However, care must be taken that cost, design and installation parameters are correct for the specific application. Jan de Jonge, managing director of Pretoria-based specialist wear resistant ceramic manufacturer Multotec Wear Lining, points this out.

    The company has achieved notable successes in the design and manufacture of linings for wear control using combinations of materials (predominantly dense alumina ceramics) and customised design, particularly in bulk handling of abrasive materials in mining, minerals processing and power generation.

    De Jonge explains that the terms advanced, engineered, technical or structural ceramics are generally used interchangeably for a group of materials, which are being increasingly used to advantage in conditions of abrasion and corrosion. These can range from boron and silicon carbide, through alumina ceramic to reaction bonded silicon nitride.

    "It must be remembered that hardness is a common factor among wear resistant ceramics. Unlike metals they are brittle and fracture with little or no plastic flow, and these properties have be taken into account in design and manufacture of wear resistant systems," he says.

    In the conveying and transport of bulk materials in heavy industry two basic types of wear predominate, namely abrasive and impact wear. Abrasive wear is frictional, caused by materials which slide or flow parallel to the surface of the wear component, while impact wear occurs when materials drop onto its surface through gravitational or centrifugal forces.

    Extensive laboratory studies of wear characteristics of ceramics for use in various applications have been carried out abroad and locally over time. These have provided a depth of data on which engineers can draw, but have also shown that there is a broad range of variables that makes ceramics design an extremely exacting technology.

    "Quantitative evaluation of wear performance under laboratory conditions is effective, but interpretation of results must be handled with care. There are certain fundamental considerations which influence both test results and actual wear performance, and great importance must be placed on selecting the correct materials for test purposes," de Jonge stresses.

    "In airborne particle erosion studies on alumina ceramics the relationship between erosion rate and fracture toughness has been found to be in approximate agreement with predictions, but the danger of interpreting such findings as indicators of fundamental relationships cannot be over-emphasised. Fracture toughness itself depends on factors such as alumina content, density and grain size, and these variables are not independent."

    It has not been possible to establish a clear relationship between wear rate and alumina content, or between hardness and wear. Even specific measurements of hardness, such as the Vickers scale, will not attest to the wear performance which can be anticipated.

    Another common method of evaluating wear performance of ceramics is sliding wear testing using pin-on-disc and ball-on-flat tribometers, of which there are many well-documented results. Most papers comment on the order of magnitude of the change in wear which can occur as a result of even a slight change in one of the test variables, such as contact load, sliding distance or sliding speed.

    It has also been shown that tribological performance can be improved through changes in composition of the ceramic material, by lowering the coefficient of friction by lubrication or taking into account tribochemical aspects of the working environment. Lubricious surfaces are known to form on silicon nitride through pre-oxidation, and in alumina through the formation of stable aluminium hydroxides, in both cases resulting in lowered wear rates.

    "By and large laboratory studies would seem to show that ceramics require zero impact and specific environments to be at all effective in combating wear. In practice, however, ceramics frequently outperform steels and polymers even in these types of high-wear applications. "

    This implies that practical experience is the most significant means to adequately assess the complexities of wear conditions and the effects of design, but taking cognisance of the Theory of Wear in the designs", de Jonge maintains, and goes on to cite a number of in-field examples which demonstrate the practical application of ceramics in industry:

    Feed chutes and hoppers - In an effort to reduce initial cost a conveyor feed chute was lined with 15 mm conventional ceramic floor tiles. Subjected to the abrasive effect of 19 mm coal the lining lasted six months. A 13 mm alumina lining which replaced it has been in service for more than 18 months and shows only minimal signs of wear.

    Vibrating centrifuge feed cone - Unlined cones were lasting less than six weeks, with resulting high downtime and replacement costs. A 6 mm ceramic lining, installed with careful attention to joint design, has reached two years' lifespan and achieved payback in less than three months.

    Pumps - A rubber-lined slurry pump was lasting only weeks, while Ni-hard cast iron achieved 1 350 hours before requiring replacement. A ceramic liner of the same design as the Ni-hard component lasted 2 250 hours, while a purpose-designed liner adapted to optimise the tribological properties of the ceramic achieved an operating life of more than 2 600 hours.

    Hydro cyclones - Wear and subsequent enlargement of spigot orifices beyond critical diameters is an important cost factor on hydro cyclones. Spigots manufactured from carefully selected and engineered ceramics are being increasingly used on hydro cyclones in particularly severe applications, with resulting savings in downtime and lost production.

    Vibrating Screens overflow, underflow and feed chutes have always had several wear and hang-up problems which have been resolved through the intelligent application of engineered dense alumina ceramics. A screen overflow chute taking minus 25 mm kimberlite has now been in service for two and half years without repair, exceeding the previous six months life considerably. Forty-five degree corner tiles have reduced the hang-up in a fine coal-handling chute which was bring the entire plant to a standstill.

    Pulverised fuel ash - Alumina ceramics have been widely used in PFA handling systems. In one case comparative wear results showed cast steel to have a wear rate of 4 mm/1 000 hours, while cast basalt wore at a rate of 2 mm/1 000 hours and % alumina at 1 mm/1 000 hours. Studies in South Africa have shown that wear rates can be considerably reduced by using staggered joints or hexagonal tiles to reduce channelling effects, and attention to design to allow gentle changes in direction in ash flow.

    Wet ash handling - Practical studies have shown that alumina have a wear resistance from 79 to 258 times better than the carbon steels previously used on ash conditioner blades.

    In another case a 30 mm cast basalt base on an ash scraper conveyor initially achieved a life of four years before wearing through. It was replaced with a 25 mm high alumina ceramic tile lining laid in a herringbone pattern to alter the angle of impact on the joints, which after four years shows only a slight "polish" and is expected to last between 15 and 20 years.

    "These examples are by no means exhaustive but they do serve to demonstrate the role ceramics can play in countering wear problems in aggressive industrial applications," de Jonge concludes.

    "While there is no single solution to the problems of wear, the most appropriate approach is to involve technical representatives of the ceramic suppliers in the design stages of equipment to ensure that, where possible, optimum performance of the ceramic is achieved within cost and specification requirements."

    Multotec Wear Linings is part of the Multotec Group which, in addition to manufacturing dense alumina ceramics, also manufactures and markets an extensive range of mineral processing equipment.

    For more information on Multotec Wear Linings (Pty) Ltd. please visit:
    http://edir.bulk-online.com/oldedirredirect/201813.htm

    Figure legend:
    Ceramic lined screen underpan with
    corner tile protection and staggered layout of tiles
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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